readme: Add instructions on how to test XBlocks

Since most XBlocks has a similar way of testing them, including it in the workbench would reduce a lot of redundancy.
@nedbat +1

@dragonfi We talked about this - the part of the instructions about the syncdb and INSTALLED_APPS are specific to mentoring. Even if mentoring isn't the only XBlock to have Django dependencies, that's not a prerequisite for XBlocks.